Notes and Comments

Species belonging to the obscura group of Drosophila have played an important role in population genetics and evolution, The best studied species of this group in North America are D. pseudoobscura and D. persimilis (Dobzhansky and Powell, 1975); in the Old World D. subobscura has received the most attention (Krimbas and Loukas, 1980). The feature of these species which has been most thoroughly studied is the genetic differentiation of populations for the chromosome inversion polymorphisms. These species show considerable quantitative (frequency) as well as qualitative (presence or absence) geographic differentiation for inversion polymorphisms. The degree to which a species is genetically structured depends on many things, among which dispersal behavior is surely important (Taylor and Powell, 1983). In a comparison of the genetic structure of these species, Ferrari and Taylor (1981) used the hierarchical analytical procedures of Wright (1978). They found that D. subobscura is geographically structured on a finer scale than is D. pseudoobscura. This was thought to be consistent with the dispersal behavior of the two species, D. pseudoobscura dispersing at a higher rate than D. subobscura (Begon, 1976; Powell et aI., 1976). However, the dispersal rates of these species had not been analyzed in a manner that permitted direct comparison, so that explanation had to be considered tentative, We present here: (a) the results of a dispersal study with D, subobscura on Mljet, Yugoslavia, done in the same manner as those done with D. pseudoobscura; and (b) a re-analysis of an early study of D. subobscura and D. obscura at Wangenerwald, near Zurich, Switzerland, done by Burla and Greuter (1959a, 1959b, 1963; Greuter, 1963). These are compared to measures of active dispersal by D. pseudoobscura and D. persimilis in California, USA. We conclude that there is no compelling evidence for significantly different dispersal rates among the four species mentioned above. Indeed, considering the variety of experimental designs employed, the measured dispersal parameters of the four species seem remarkably similar. Thus differences in population structuring among these species must be due to factors other than active dispersal behavior (sensu Dobzhansky, 1973).

seem remarkably similar. Thus differences in population structuring among these species must be due to factors other than active dispersal behavior (sensu Dobzhansky, 1973).

Mljet Measurements
The previously unpublished data presented here was collected in August 1982 on the island ofMljet, located in the Adriatic Sea off the coast of Yugoslavia. The procedures were similar to those described in Powell et al. (1976). On a release day fermenting fruit baits were exposed late in the afternoon and flies were collected until nearly dusk. Shortly before dusk the baits were removed from the field, captured flies were marked with fluorescent dusts (Crumpacker, 1974), and released at a single release site. Late in the afternoon on the day after a release, baits were brought to the field and placed in two lines at right angles intersecting at the release site. Ten baits at 20-m intervals constituted each of the four arms and a single bait was placed at the release site. Flies captured at each bait were examined that evening for fluorescent dusts, using UV light with a microscope. On subsequent releases differently colored dusts were used, The procedures developed by Wright (1978;Wright, 1943, 1947) were used to estimate dispersal.
When movement is random and uniform, the density of flies that are released from a single location will assume a bivariate normal distribution, centered on the release site and with a variance that increases proportionately with time (Pielou, 1977;Wright, 1978). The single parameter which best characterizes this distribution is the radial variance, equal to the mean square distance of flies from the release point. Wright (1978) and Dobzhansky and Wright (1943) have shown how this can be calculated from release-recapture data of the type collected here. We refer to this below as the Wright mean square (WMS) dispersal distance.
The results of two releases on Mljet are shown in Table 1. All the recaptured obscura flies were identified as to species. Too few marked flies were recaptured two or more days after release for meaningful interpretation, so only dispersal for one day after release will be considered. A summary analysis of these data is shown in Table 2. The two releases gave nearly identical estimates of daily dispersal: the standard deviation of the distribution is 90 m, Wright's mean squared radial variance is 1.6 x 10' m-, and the average radial distance traveled is between 100 and 120 m. Burla and Greuter (1959a, 1959b and Greuter (1963) collected extensive dispersal data on D. subobscura and D. obscura at several locations near Zurich, Switzerland. They did not analyze the data in such a manner as to allow comparison to the present study or to the estimates made by Dobzhansky and co-workers. At one of their locations, Wangenerwald, the placement of their traps was similar to that on Mljet. There they made three releases ofeach species. Their procedure differed from the present one in two important ways: laboratory-reared, rather than native, flies were the release group; and recapture baits were at 10 m, rather than 20 m, intervals. We present in Table 2 the various dispersal parameters estimated using the same analytical procedures as for Mljet, For both species, the dispersal parameters are only marginally smaller than those we determined on Mljet. Ifwe discount the last release, which had low sample sizes for both species, the average radial distance for D. subobscura is approximately 85 m and approximately 100 m for D. obscura. These are only slightly smaller than the 110 to 120 m obtained for D. subobscuraon Mljet. It is probable that the Swiss studies yielded somewhat lower estimates because they were done with laboratory-reared flies, which Direction are thought not to disperse quite so far as native flies (powell et aI., 1976).

Wangenerwald Measurements
Burla and Greuter's data include recaptures for up to 12 days after release, from which it is possible to examine the temporal growth in dispersal. Ifdispersal is random (Brownian motion) and temporally constant, then we expect a linear relationship between WMS distance against time. The relation they actually observed is plotted in Figure 1. For the first 2 days the increase of estimated WMS is approximately linear with time, but after this time there is a levelling off, with the estimated WMS asymptotic to 2 x 10' m-, As we discuss below, this leveling is probably an artifact of the length of the trap lines.

DISCUSSION
The object of this study was to measure the dispersal of D. subobscura and to compare it to that of D. pseudoobscura. Many procedural differences between the earlier studies and those reported here stand in the way of a direct comparison. In some cases (e.g., Crumpacker and Williams, 1973;Begon, 1976;Loukas and Krimbas, 1979) the differences seem so large that comparison is impossible, as a result ofdifferent patterns oftrap placement, among other things. But some studies do seem sufficiently similar that adjustment is possible, and the desired comparisons can legitimately be made, so long as procedural differences are recognized and are accounted for.
Important among these procedural differences are the lengths ofthe lines along which traps were placed and the source ofthe flies whose dispersal was measured. Dobzhansky and Wright (1943) and McInnis et al. (1982) have emphasized that the length of trap lines is an especially important factor. This is because many of the dispersal parameters are calculated from the squared distance at which recaptures are made. A few far dispersers can contribute substantially to the parameter. One way to control for differences in trap line length is to artificially truncate the lines when doing the analysis, so that they are equal from study to study. For example in Table 2 we summarize the study by Powell et al. (1976), who used trap lines 800 m long. To compare their results to those for D. subobscura, we recalculated the dispersal parameters using these data to only 200 m. The parameters then became less than halfthose calculated using the 800 m line. The average radial distance decreases from 312 m to 132 m. This latter figure compares favorably with the 110 to 120 m found on Mljet and is somewhat greater than the 85-100 m calculated from Burla and Greuter's data. Wright (1943, 1947) did releases with laboratory-reared D. pseudoobscura, and their data for 300 and 200 (truncated) meter trap lines are shown in Table 2. The average radial distance for 200 m lines is 95 m, very close to Burla and Greuter's estimate made with laboratory-reared flies. Though it would be desirable to have confidence limits on these estimates of dispersal, that would be difficult at best West 7-1 10-1   Wright (1978) and Wright (1943,1947) (Powell et al., 1976). Considering the many procedural and environmental differences among the studies summarized in Table 2, it is remarkable that the estimates, when comparably analyzed, are so uniform. When trap lines are held constant and possible effects of laboratory rearing are considered, virtually all of the estimates reported here for D. subobscura and D. obscura fall within the range of estimates reported for D. pseudoobscura and D. persimilis. We conclude that the species of the obscura group which have been studied do not differ greatly in the distances they actively disperse.
This uniformity of dispersal behavior might appear to be incompatible with the previously documented variation in dispersal behavior dependent upon fine scale ecological changes (Burla and Greuter, 1959;Dobzhansky et aI., 1979). This is not necessarily so. Drosophila movement is dependent on many factors such as vegetation, temperature, light intensity, and humidity. These factors vary on a fine scale in the heterogeneous woodlands where the dispersal studies were conducted. However, when considered over hundreds of meters for 24 h or more, this microscale variation in behavior appears to "average out." Thus the relative uniformity in dispersal parameters from study to study and across species when dispersal is measured over long distances, is not incompatible with fine scale variation in behavior.
It may be that dispersal is substantially different in those instances where ecological differences do not "average out" but are instead unusually benign or harsh over very long distances. Jones et al. (1981), for example, have observed D. pseudoobscura in the desert, where flies may disperse several kID in a single day. It must be recognized that the flies in Jones et al.'s study were transported 100 kID or more from their site of origin and it appears that these populations are transient, in that they do not survive throughout the year (Moore and Moore, 1984). Consequently, it is not clear whether their findings pertain to flies residing in more benign circumstances. Nonetheless, the possibility must be kept open that sometimes ecological settings do not average out and that substantial differences in dispersal may sometimes occur and be important.
The importance of trap line length, and the need to adjust for it, is illustrated in Figure I ment is random and temporally uniform, then one expects a linear relationship between WMS and time (Dobzhansky and Wright, 1943;Pielou, 1977). The predicted relationship is apparent in the data for at best only two days, after which there is a leveling. This occurs not because the flies cease dispersing, but because the trap lines are not long enough. If the marked flies were uniformly distributed over trap lines of length r, then it is easily shown that we expect a WMS square of('12)'>, or 20,000 m-for a trap line of 200 m. As will be noted from Figure  1, this is very close to the values at which these graphs level off. In other words, by three or four days after release, the marked flies' distribution is such that the mean square is indistinguishable from that of a uniform distribution, and it is impossible to measure any further dispersal unless movement is not Brownian. (This may be the case with D. obscura, in which the leveling appears to occur somewhat higher than 20,000 m-.) Any attempt to measure dispersal is bound to cause some disruption of the population. In particular, the methods used here involve a high density at the release site and the placement of artificial baits in an otherwise natural area. To the best of our ability to measure them, such disturbances are small (Powell et aI., 1976). But even if disturbed, assuming the species are disturbed similarly, then our interpretation of the results would not be affected. Dobzhansky (1973) has distinguished between intermigration by "active dispersal," as measured here, and by "passive transport," whereby a few individuals may be transported long distances by external agencies such as high winds or motor vehicles. When Dobzhansky first began his studies of D. pseudoobscura in the American Southwest there was little tourism in the remote sites he collected SUMMARY Results ofa study ofdispersal by Drosophila subobscura on the Adriatic island of Mljet are presented. A previous study by Burla and Greuter on D. subobscura and D. obscura is re-analyzed for comparison. Data from other studies on these and related species are reviewed. Our major conclusion is that all obscura group species so far studied have similar dispersal rates. If data from the previous studies are standardized to a trap line length of200 m, the mean squared distance traveled in one day is 1-2 x 10 4 m-, and the average radial distance in one day is about 100 m. If extrapolated to longer trap lines, the average radial displacement per day is approximately 310 m/day for all species considered.
Differences in hierarchical population structure among the species cannot be accounted for by differences in active dispersal.

ACKNOWLEDGMENTS
We thank the Fulbright Foundation and the National Science Foundation for financial support, and L. B. Klaczko for useful comments on this manuscript.
from and from which the population structure of D. pseudoobscura was measured. Since that time tourism has increased greatly, so that passive transport is likely to be much more important. There is some evidence that this is responsible for inconsistent measures of population structure for recessive lethals in D. pseudoobscura (Taylor and Powell, 1983), but we are aware of no such evidence for gene arrangements. Selection for this trait is probably so strong, especially in D. pseudoobscura, that a few especially long dispersers are unlikely to be important. For the population structure ofgene arrangements it seems likely (though unproven) that movement by active dispersal completely dominates that by passive transport.
While there is some variation in the adjusted parameters in Table 2, most is probably within the range of experimental error for such studies. Considering all the variation in methodology in the studies summarized here, this consistency is particularly noteworthy and lends credence to our conclusion that these species display remarkable similarity in dispersal behavior. Thus if these species differ in their microevolutionary dynamics (Ferrari and Taylor, 1981;Taylor and Powell, 1983), there is no evidence that differences in active dispersal behavior are involved.